2 grounding/bonding questions

Discussion in 'Electrical Forum discussion & Blog' started by Homeownerinburb, Feb 14, 2014.

  1. Homeownerinburb

    Homeownerinburb New Member

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    1) Just had a short debate with my local inspector on a job I am bidding. The house is from 1947 and the wiring is all in steel flex conduit.

    He told me that flex conduit cannot be used as the ground for more than 6'. I always understood that to apply when the ampacity of the circuits inside were greater than 20amps. For conventional branch circuits around a house, I had understood that the flex conduit was indeed the ground, no matter how far from the panel it runs.

    2) May well be installing a 50 amp sub panel in an outbuilding. I don't need to run a ground/bond back to the main panel, or has that changed of late? I expect to drive two ground rods and call it good.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Flex steel conduit makes for a lousy ground. I wouldn't use it for that even it if was allowed. The house I grew up in has that, and corrosion over the years just makes it totally useless.

    I'd run a ground back to the main panel. I do not think it will pass current codes without one. You still need the ground rods on the outbuilding. The grounding is for primarily lightning surge control. The actual ground wire provides a safety path for tripping the fuse or CB...a ground rod is not sufficient for that.
  3. cacher_chick

    cacher_chick Test, Don't Guess!

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    AC listed cable is approved for the equipment ground, FMC or LFMC is only approved for 6 feet when used with grounding fittings.

    Sub panel will need a ground back to the main panel AND grounding rods at the sub.
  4. bluebinky

    bluebinky Member

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    The sub panel will also require a neutral bus that is not bonded to ground -- only connect ground and neutral at the main panel.
  5. jwelectric

    jwelectric Electrical Contractor/Instructor

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    250.32(A)(A) Grounding Electrode. Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system installed in accordance with Part III of Article 250. The grounding electrode conductor(s) shall be connected in accordance with 250.32(B) or (C). Where there is no existing grounding electrode, the grounding electrode(s) required in 250.50 shall be installed.

    250.32(B)(1) Supplied by a Feeder or Branch Circuit. An equipment grounding conductor, as described in 250.118, shall be run with the supply conductors and be connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The equipment grounding conductor shall be sized in accordance with 250.122. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).
  6. hj

    hj Moderator & Master Plumber Staff Member

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    That reads like something written by a lawyer, or an engineer.
  7. DonL

    DonL Jack of all trades Master of one

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    Written by a engineer and a lawyer would not understand it.
  8. bluebinky

    bluebinky Member

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    Written by a committee of Engineers based on current engineering best practices and more than a century of field experience from people like HJ and JW.

    The wording is the way it is to keep Lawyers from doing electrical installations and then successfully arguing that they did it correctly.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Words, and those especially in English, often have slightly different connotations because most of them were absorbed from other languages. As a result, writing something that is as unambiguous as possible, can end up sounding strange since most people do not talk that way. The goal, not always achieved, is to make it as unambiguous as possible so that once you do understand it, there is only one 'right' way to perform the task.
  10. Homeownerinburb

    Homeownerinburb New Member

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    JW. You did not actually answer either of my questions, I think.

    1) Does the steel flex conduit count as "an equipment grounding conductor" still? Obviously, back in 1947, it did. And if so, I would be grandfathered in.

    But if it is no longer permissible, even on 20 amp or smaller circuits to rely on the flex conduit as the ground path, and that a separate conductor needs to be pulled, in say, a re-wire, then that is news to me, and the inspector for the extensive rewire I did a few months back in an adjacent city certainly missed the boat on calling me out on it. Granted, that was built in '28 with black gas pipe (ships were wood and men were iron). He had me installing arc faults but no separate conductor. I certainly found low resistance to ground in my tests.

    So, is it acceptable to use the conduit as a ground, or need I run a specific (green) conductor?

    1a) And as a separate question: nearly none of the circuits in the house I am looking at just now are showing a valid ground at the receptacles. The ground at the main panel reads fine. In a house that is going on 70 years of age, is it a reasonable assumption that the interface of the conduit, the connectors and the boxes has all so completely corroded that no reliable ground is available.

    I am writing a document on behalf of the buyer to squeeze concessions from the seller for upgrades, and I would as soon not be talking out of my ass.

    2) For the out building sub panel, I know that I need ground stakes, and that that has been the standard for decades or more.

    But is there a NEW requirement for a grounding conductor to bond the grounding buss bar in the sub panel to the ground/neutral buss bar in the main panel?
  11. Reach4

    Reach4 Well-Known Member

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    What are the implications of this?
  12. hj

    hj Moderator & Master Plumber Staff Member

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    quote; What are the implications of this?

    On the surface, it would appear that the grounding element just terminates in the air because it cannot be connected to anything.
  13. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Location:
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    Homeowner

    If the armor of AC cable has a bonding strip that is included in the folds of the armor then the cable armor can be used as an equipment grounding conductor. Back in 1947 there was no requirement to install an equipment grounding conductor with the circuit conductors so to presume that the armor is a grounding path would be futile.

    As a sworn code enforcement official should someone in my jurisdiction be replacing receptacles in such an installation as you describe then the two wire receptacles would be replaced with GFCI devices and labeled no EGC as well as Arc Fault protection for the circuit.

    For several decades now an equipment grounding conductor has been required for feeders to a remote building if there is any other electrical path between the two buildings such as telephone or other utility or even a chain link fence that is attached to both buildings. I have never in 47 years not installed an EGC with the feeders as this is the low impedance path to cause the breaker to trip or the fuse to blow. Earth is not a low impedance path and will not clear a fault.

    Also Part II of Article 225 mandates that one of four minimum feeder circuits to be installed and 50 amps is not one of them. If the feeder is going to be larger than 30 the next smallest size allowed is 60 amps, see 225.39 for more information on this subject.


    Reach4

    The groundED conductor is the neutral as outlined in Article 200 and the groundING conductor is the fault clearing conductor outlined in Article 250
  14. Homeownerinburb

    Homeownerinburb New Member

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    Location:
    Los Angeles, CA USA
    JW.

    Thanks. I'm not referencing AC, but rather conduit with separate conductors pulled in.

    But, yes, I had been expecting to pull a dedicated ground/bond wire to the sub panel.
  15. jwelectric

    jwelectric Electrical Contractor/Instructor

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    I am sorry but I took this statement to mean you were talking about what everyone calls the old BX cable which was AC cable
  16. Homeownerinburb

    Homeownerinburb New Member

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    No, neither BX nor AC, which is not very common out here. Steel Flex conduit with conductors pulled in? Oh yeah! The vast majority of what I see. I like it pretty well.

    Now, is my inspector correct or otherwise in telling me that flex conduit is not permissible for more than 6' without a dedicated ground conductor, if the circuit(s) inside do not exceed 20 amp? Or, is flex conduit of an ampacity to clear a fault up to 20 amp?

    Oh, and another cable I saw ONCE. In a house built during WW2. A form on NM. A plastic sheath with one insulated conductor and one bare. A hot and a grounded. No grounding.

    And what do you call that nasty tarry stuff that is also NM, but has two insulated conductors but no ground? It has a wrapping of fabric. It looks like an invitation to a conflagration to me.
    Last edited: Feb 15, 2014
  17. jwelectric

    jwelectric Electrical Contractor/Instructor

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    (5) Listed flexible metal conduit meeting all the following conditions:
    a. The conduit is terminated in listed fittings.
    b. The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less.
    c. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground-fault current path does not exceed 1.8 m (6 ft).
    d. If used to connect equipment where flexibility is necessary to minimize the transmission of vibration from equipment or to provide flexibility for equipment that requires movement after installation, an equipment grounding conductor shall be installed


    This type of cable had rubber insulation around the conductors. Below is a link to a very good power point with loads of information concerning this type of wiring.
    http://www.nfpa.org/~/media/Files/proceedings/dini_presentation_-_resa_project.pdf
  18. Homeownerinburb

    Homeownerinburb New Member

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    Location:
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    Thanks. So, no. Flex conduit no longer serves as a grounding conductor.
  19. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Location:
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    To confuse Flexible Metal Conduit with the old AC cable is easy to do. The telltale sign is the paper wrapping of the conductors.

    FMC has never been a GEC even though using a meter will show continuity.
    The issue is how much fault current the armor will carry without failing.
    Due to the manner in which the armor is wrapped what happens is the armor will arc and burn into.
  20. Homeownerinburb

    Homeownerinburb New Member

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